Modulated composition flame photometer

ABSTRACT

This invention relates to an improved flame photometer. The gas to be analyzed is modulated in composition by means of a reference gas devoid of the component whose concentration is sought. This reaction is accomplished with a modulator cavity partitioned by a flexible membrane having a ferrite magnet or other magnetic material attached thereto. An electromagnet imparts a sinusoidal displacement to said ferrite magnet producing peak tidal flows of said sample gas and said reference gas alternately to a burner. An optical filter passes specific spectral components of the resultant luminosity to a photomultiplier which converts the light flux to an electrical signal. This resultant signal is electrically correlated with the modulating drive signal to obtain the amplitude of the periodic component of flame intensity occurring at the modulating frequency.

United States Patent 151 Green June 19, 1973 1 1 MODULATED COMPOSITIONFLAME PI-IOTOMETER Joseph A. Green, Adelphi, Md.

[22] Filed: Oct. 28, 1971 [21] Appl. No.: 193,288

[75] Inventor:

[52] US. Cl. 356/187, 356/85, 356/87,

[51] Int. Cl. G01j3/48 [58] Field of Search 356/85, 87, 187,

[56] References Cited UNITED STATES PATENTS 3,620,628 11/1971 Yasuda eta1. 356/87 3,625,614 12/1971 Herrman et al 356/87 OTHER PUBLlCATlONSAnalysen der AtomabsoptionsFlammenphotometrie by l-lerrman, Z. lnstr. 75(1967), Pg. 101-! ll Piezoelectrically Induced Selective Flame SignalModulation by Mossotti et al.,'Applied Spectroscopy,

Vol. 25, No.3, 1971 pg. 331-341 Primary Examiner-Ronald L. WibertAssistant ExaminerPaul lQQodwin A ttorney-C. Cornell Remsen, Jr., WalterJ. Baum, Paul W. Hemminger et al. v

[57] ABSTRACT This invention relates to an improved flame photometer.The gas to be analyzed is modulated in composition by means of areference gas devoid of the component whose concentration is sought.This reaction is accomplished with a modulator cavity partitioned by aflexible membrane having a ferrite magnet or other magnetic materialattached thereto. An electromagnet imparts a sinusoidal displacement tosaid ferrite magnet producing peak tidal flows of said sample gas andsaid reference gas alternately to a burner. An optical filter passesspecific spectral components of the resultant luminosity to aphotomultiplier which converts the light flux to an electrical signal.This resultant signal is electrically correlated with the modulatingdrive signal to obtain the amplitude of the periodic component of flameintensity occurring at the modulating frequency.

3 Claims, 2 Drawing Figures [111 3,740,154 [451 June 19, 1973 UnitedStates Patent [1'91 Green s R an m W h Pu mu r M me u m m #0 o A Y I. 5z 0/ m y L 3 a m R m ma A6 P Wm M M LN 5 mmm M Z w mm. m on P mm R 8! NH m 1 n P w MA m CM 0 FLA/1E CHAMBER E Z Wan/us GAS INPUT MODULATEDCOMPOSITION FLAME PHOTOMETER BACKGROUND OF THE INVENTION This inventionrelates to a flame photometer and more particularly to a modulatedcomposition flame photometer.

An ultimate limit to the sensitivity of existing flame photometers isusually established by random fluctuations in the luminosity of thebackground flame. Tem poral variations of the spectral components of thebackground flame that pass through any optical filters that are usedproduce responses at the output of the photodetector that can beinterpreted erroneously as being due to variations in the composition ofthe gas being analyzed. Current practice invokes means for suppressingthe time invariant component of this undesired flame response, but doesnothing to combat the fluctuating component, including any long termdrift that may be present.

SUMMARY OF THE INVENTION It is an object of the present invention toprovide a flame photometer in which the sensitivity is enhanced byreducing the effect of luminosity variations.

According to a broad aspect of the invention there is provided animproved flame photometer of the type wherein there is provided a flamechamber containing a burner for burning a stream of sample gas to beanalyzed, an optical filter for passing specific spectral components ofthe luminosity, and a photomultiplier for converting light flux to anelectrical signal wherein the improvement comprises a source of areference gas, means for composition modulating said sample gas streamwith said reference gas and means for measuring the amplitude of theperiodic component of the electrical signal that occurs at themodulating frequency.

The above and other objects of the present invention will be moreclearly understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of a flamephotometer employing gas composition modulation and the associateddetector circuitry; and

FIG. 2 shows the burner assembly employed in the inventive flamephotometer.

DESCRIPTION OF THE PREFERRED EMBODIMENT The object of this invention isattained by composition modulating the sample gas stream supplied to theburner flame at a frequency of several tens of cycles per second andelectronically correlating the output of the photodetector with themodulator drive signal. This can be more clearly explained withreference to FIG. 1.

The sample and reference gas sources are assumed available at aboveambient atmospheric pressure from high impedance source; that is, thesegases are forced into modulator I through inputs 2 and 3 at constant.flow rates designated 0,, and Q,.,- respectively and then into flamechamber 4 operated at essentially atmospheric pressure.

The reference gas such as nitrogen, and sample gas inflow rates Q, and Oto the modulator are adjusted to be approximately equal using knownmethods. A diaphragm 5 divides the modulator cavity and is given asinusoidal displacement with an amplitude just sufficient or slightlyless than that required to produce peak tidal flows equal to thereference gas or sample gas inflow rates taken individually. Themodulator cell 1 consists of an approximately cylindrical cavitypartitioned by flexible diaphragm 5. A small ferrite magnet or othermagnetic material 6 is mounted in the center of diaphragm 5 in orderthat the latter can be electromagnetically driven, in the manner of aloudspeaker diaphragm, by means of modulator driver 7 controllingelectromagnet 8. The electromagnets are external to the cavity itself,precluding any need for low friction gas seals. The pneumatic sourceimpedance of the sample and reference gas supplies is high with respectto the exit impedance of the modulator chambers. Because of this,diaphragm motion does not significantly disturb the gas inflow rates butonly influences the output flow from the modulated chambers.

When the above conditions are met, the gas stream from the modulatorchamber 1 to the flame chamber 4 will vary in composition in thefollowing manner: at one instant in the modulation cycle, thecomposition will essentially correspond to that of the input sample gasstream alone. One-half cycle later, the composition will essentiallycorrespond to that of the reference gas stream alone. It should be notedthat throughout the entire modulation cycle, despite the approximatelysinusoidal composition modulation, there is no modification to theoverall flow exiting from the modulator chamberto the burner 9 in flamechamber 4. This precludes undesired modulation of the flame luminosityby virtue of changes in the dynamics of the flame. Hydrogen and oxygenare fed to burner 9 through inputs l8 and 19 respectively to supportcombustion.

If the sample and the reference gases are identical in composition, theluminosity of the flame will be independent of the phase of themodulation cycle. However, should the sample gas contain a substance.that will increase the luminosity of the flame, such as sulphur orphosphorous compounds, the flame intensity will contain a periodiccomponent occurring at the modulation frequency.

The optical components shown in FIG. 1 are essentially identical tothose used in the Melpar/Tracor photometer. Light from flame chamber 4is filtered via optical interference filter 10 said interference filterbeing of the type manufactured by Baird Atomics to Tracorspecifications. In this manner, only light of specific frequencies,corresponding to the burning of certain gases in the flame chamber, willpass through filter 11 and excite a photomultiplier assembly 11., suchas that used in the Melpar photometer. The detection of a different gascan be accomplished by changing optical interference filter 10 to allowpassage of a different frequency to the photomultiplier.

The burner assembly 9 consists of a set of triaxially disposed tubesterminating somewhat below the center of the flame chamber. The centraltube is partitioned longitudinally into two halves so that a total offour independent pneumatic paths extend into the region of the flame.This'is shown in more detail in FIG. 2.

Hydrogen flowing through the outer annulus 15 and oxygen flowing throughthe contiguous annulus l6 sustain a flame while sample and reference gasflows are each admitted to the flame by means of one of the channelsprovided by the partitioned central tube 17.

The output of photomultiplier 11 is coupled to synchronous detector 12via an AC coupled amplifier 13. Synchronous detection of thephotodetector output signal with a signal obtained from modulator driver7 affords a convenient means for measuring the amplitude of the desiredsignal in the presence of random fluctuation. Signal-to-noise ratioenhancement can be purchased at the expense of diminished response timeby recourse to extended post-detection integration times.

Such a system as just described would be useful for example, in theanalysis of the eluent of a gas chromatograph. Some modification isrequired, if the photometer is to be used with ambient atmosphericpressure sample and reference sources. In this case, a pump downstreamof the combustion chamber would be required to maintain flow, and adevice such as a filter or scrubber or external source would be requiredto supply clean reference gas.

For a quantitative description of the modulator oper ation let Q,,,sample gas inflow rate (cc/sec) Q reference gas inflow rate (cc/sec) Qsample gas outflow rate from modulator (cc/sec) Q reference gas outflowrate from modulator (cc/sec) Q composite gas flow rate to flame (cc/ ec)Q, trace gas flow rate to flame (cc/sec) C, concentration of trace gasin sample input C concentration of trace gas in flow to flame V peaktidal amplitude produced by modulator (i) angular frequency with whichmodulator is driven (radians/- sec) The gas flow rate issuing from thesample side of the modulator can be simply expressed:

I v Qnu Q, VHJCOSUJI The corresponding flow rate from the reference sideof the modulator is:

Qru Qri Vwcoswt Because the tidal flows on opposite sides of themodulator diaphragm are opposite in phase, the tidal flow terms in (l)and (2) differ in sign. The composite gas flow after the combiningmanifold is:

Qc Qso Qro Qsi Qri Thus the gas flow rate to the flame is seen to beconstant and independent of the tidal flow.

Considering the trace gas flow to the flame:

Qt inQso in (Qxi Vwcoswt) and, dividing (4) by (3) to obtain the tracegas concentration supplied to the flame:

oul in [Qsi /Qsi Qri] Physically it is desirable that VwcosmtsQ andVmcoswt Q so that undirectional flow is always maintained to the burner.Otherwise, the composition within the modulator chambers would be afunction of modulation amplitude, dead volume within the modulator anddelivery system, etc., and the achievable modulation depth would bereduced. Optimum modulation occurs when:

Qsi Qri Va) in which case the trace gas composition supplied to theflame is fully modulated. It should be noted, however, that smalldepartures from (8) can be tolerated without significant degradation ofperformance so long as conditions (6) and (7) are still satisfied.

It is to be understood that the foregoing description of specificexamples of this invention is made by way of example only and is not tobe considered as a limitation on its scope.

I claim:

1. An improved flame photometer of the type wherein there is provided aflame chamber containing a burner for burning a stream of sample gas tobe analyzed, an optical filter for passing specific spectral componentsof the luminosity and a photomultiplier for converting light energy toan electrical signal wherein the improvement comprises:

a source of a reference gas;

means for composition modulating said sample gas stream with saidreference gas, said modulating means comprising:

a cavity having a first input coupled to said source of reference gas, asecond input for receiving said sample gas and first and second outputsfordelivering said reference gas and said sample gas to said burner;

6 a flexible diaphragm partitioning said cavity and a ferrite magnetmounted in the center of said diaseparating said first input and saidfirst output phragm; from said second input and second output; and anelectromagnet located outside said cavity for dismeans for imparting tosaid diaphragm a sinusoidal placing said ferrite magnet; and

displacement to produce peak tidal flows of said 5 means for controllingsaid electromagnet to impart sample gas and said reference gas to saidburner sinusoidal displacement to said ferrite magnet. equal to thereference gas and sample gas inflow 3. An improved flame photometeraccording to claim rates; and 2 wherein said measuring means includes:means for measuring the amplitude of the periodic a synchronous detectorhaving a first input coupled electrical signal corresponding to flameintensity 10 to the output of said photomultiplier and having avariation occurring at the modulating frequency second input coupled tothe output of said means 2. An improved flame photometer according toclaim for controlling. 1 wherein said imparting means includes:

1. An improved flame photometer of the type wherein there is provided aflame chamber containing a burner for burning a stream of sample gas tobe analyzed, an optical filter for passing specific spectral componentsof the luminosity and a photomultiplier for converting light energy toan electrical signal wherein the improvement comprises: a source of areference gas; means for composition modulating said sample gas streamwith said reference gas, said modulating means comprising: a cavityhaving a first input coupled to said source of reference gas, a secondinput for receiving said sample gas and first and second outputs fordelivering said reference gas and said sample gas to said burner; aflexible diaphragm partitioning said cavity and separating said firstinput and said first output from said second input and second output;and means for imparting to said diaphragm a sinusoidal displacement toproduce peak tidal flows of said sample gas and said reference gas tosaid burner equal to the reference gas and sample gas inflow rates; andmeans for measuring the amplitude of the periodic electrical signalcorresponding to flame intensity variation occurring at the modulatingfrequency.
 2. An improved flame photometer according to claim 1 whereinsaid imparting means includes: a ferrite magnet mounted in the center ofsaid diaphragm; an electromagnet located outside said cavity fordisplacing said ferrite magnet; and means for controlling saidelectromagnet to impart sinusoidal displacement to said ferrite magnet.3. An improved flame photometer according to claim 2 wherein saidmeasuring means includes: a synchronous detector having a first inputcoupled to the output of said photomultiplier and having a second inputcoupled to the output of said means for controlling.